Electro-optic devices are essential components of optical communication systems. By electrically changing the refractive index of material in an optical pathway, they can switch, attenuate or modulate an optical signal.
A commonly used electro-optic device employs an electrical field to control a waveguide path within an electro-optic material such as an electro-optic organic polymer or a thin lithium niobate crystal. The waveguide path is formed locally, as by doping or otherwise altering the material to increase the refractive index. The electrical field applied to the waveguide can controllably vary the refractive index in the path.
FIG. 6A illustrates a conventional electro-optic modulator comprising an electro-optic material 61 including, at the surface, an optical waveguide path 62 having a greater refractive index than the surrounding cladding material (61, 68 ). A signal electrode 64 and a ground electrode 65 are provided for controlling the electrical field in the region of waveguide path 62. A dielectric layer 68 that is relatively transparent at the optical frequency of interest and has a refractive index smaller than that of the waveguide can be disposed between the waveguide 62 and the electrodes to reduce absorption of guided light by the electrode metal. A traveling wave signal source 67 is connected to the electrodes, as by a coaxial cable 69A. Similarly, a terminal resistor 70 can be connected by coaxial cable 69B. The end surfaces of the waveguide path 62 can be connected to optical fiber segments 71 by couplers 72.
FIG. 6B shows a cross section of the FIG. 6A device along the line A–A′. The electro-optic material 61 can be an electro-optic polymer, a ferroelectric oxide, or a semiconductor. Suitable electro-optic polymers are described in References 1 and 2 cited at the end of this application herein “[1,2]”. An exemplary ferroelectric oxide is a thin crystal of lithium niobate (LiNbO.sub.3) cut so that an x-axis of the crystal extends in a longitudinal direction and a z-axis extends in a direction of thickness. Suitable semiconductor materials include gallium arsenide and indium phosphide. The waveguide path 62 is configured in two arms as a Mach-Zehnder interferometer.
In operation, an input optical beam is split between the two arms of the interferometer. The two beams interfere when the arms subsequently couple together. To modulate the beam, a voltage is applied to at least one arm to electro-optically change the refractive index in the arm. If the voltage is properly chosen, it can ensure that the beams destructively interfere and produce no output at the output waveguide which effectively defines an aperture. Thus, for example, an applied electrical signal switched between the voltage required for destructive interference and a different voltage can modulate the transmitted optical beam at a high rate.
To minimize electrical loss, the electrodes have conventionally been made relatively thick (several micrometers) in order to avoid concentrating the electrical power density in a small cross section of material. Such concentration can produce high electrode propagation loss. With high loss, the drive voltage is quickly attenuated along the optical axis of the modulator with the result that only an initial portion of an arm, and not its full length, is effective for modulation. Moreover, this attenuation is aggravated at high frequencies, reducing the highest operative pulse modulation and thus the device bandwidth.
Accordingly, there is a need for improved electro-optic devices having higher operative pulse modulation and increased bandwidth.